1. Field of the Invention
The present invention relates to a low alloy steel to be used chiefly under a corrosive environment, and in particular, the invention is suitable for application to turbine members such as large-sized turbine rotors for geothermal power generation.
2. Description of the Related Art
In the geothermal power generation, while a steam temperature is low as about 200° C., the steam contains corrosive gases such as hydrogen sulfide. In view of this fact, in turbine rotor materials for geothermal power generation, a high-temperature creep strength which is required for thermal power generation is not necessary, but corrosion resistance, tensile strength at room temperature, yield strength, and toughness are regarded as important. In such a low-temperature range, an NiCrMoV steel with excellent toughness containing from 3 to 4% by mass of Ni is usually used. However, the steel type containing a large amount of Ni involves such a defect that SCC (stress corrosion cracking) is easily caused. Accordingly, materials having enhanced toughness are used for rotors for geothermal power generation on the basis of a 1% CrMoV steel (nominal) which has been developed chiefly as a high pressure rotor or medium pressure rotor for thermal power generation. Since the 1% CrMoV steel for a high pressure rotor or medium pressure rotor for thermal power generation is used in a high-temperature range of 350° C. or higher, while the creep strength is high, large toughness is not necessary. However, in order to use such a 1% CrMoV steel for a geothermal rotor, it is necessary to enhance the toughness. For that reason, the following patents are proposed (see JP-A-52-30716, JP-A-55-50430, JP-A-61-143523 and JP-A-62-290849).
In recent years, following an increase of the power generation capacity, the increasing size of the geothermal power generation turbine rotor is being advanced, and the 1% CrMoV steel, which has been conventionally used, becomes unable to cope with the increasing size of the turbine rotor. This is because the 1% CrMoV steel is a steel type which is difficult to perform the increasing size from the viewpoints of hardenability and segregation resistance. For example, in a case of increasing a size of the 1% CrMoV, there are involved such problems that the cooling rate in a central part of the rotor is largely decreased, and ferrite is precipitated, resulting in a decrease of the toughness; and that the C concentration occurs on the side of a feeder head for steel ingot, resulting in a possibility that quenching crack is caused by water cooling at the time of quenching. In JP-A-52-30716, JP-A-55-50430 and JP-A-61-143523, though the toughness of the 1% CrMoV steel is improved, various problems to be caused due to the increasing size are not taken into consideration, and there is a concern that the toughness is decreased due to a decrease of the cooling rate. In JP-A-62-290849, though a decrease of the cooling rate to be caused due to the increasing size is taken into consideration, the problem regarding the C concentration on the side of a feeder head for steel ingot in the case of manufacturing a large-sized steel ingot is not taken into consideration, and there is a concern that the segregation resistance at the time of manufacturing a large-sized steel ingot is deteriorated.